Lasers are a common and important mechanism for cutting, marking, welding, and surface modifying substrates such as metals including stainless steel. The term "Laser" is an acronym for Light Amplification by Stimulated Emission of Radiation. The most common,stimulating media are carbon dioxide gas and neodymium yttrium aluminum garnet (Nd:YAG).
Maximizing laser productivity depends on output power, laser beam quality, and ease of operation of the device for its intended purpose. One of the more common reasons for poor laser performance is the result of contamination of the laser apparatus, particularly the focusing lens. Sources of laser contamination include impurities in the laser gases or in the nozzle pressurizing gas, vacuum-system leaks, back-streaming of vacuum pump oil into the laser cavity, sputtering of metal atoms from electrodes, fingerprints and vacuum grease, and contaminants present where optics are stored, and the like.
Many laser systems use some type of nozzle in front of the final focusing lens. The nozzle delivers a pressurized gas to the workpiece and is designed to prevent debris from spraying onto the focusing lens. Contamination of the focusing lens is one of the prime reasons for poor laser performance and downtime of laser operations. However, no nozzle system is perfect and some debris, fume or backspatter will occasionally reach the lens.
More recently laser systems have employed a protective window to protect the focusing lens against contamination. The protective window is positioned in front of the focusing lens and both optics are contained within a lens assembly. This system is disadvantageous because the protective window requires modification or disassembly of the lens assembly to replace or renew the protective window which can alter the focal length of the laser beam. Such a system is frequently impractical because the lens holder is not designed to accommodate a protective window.
Lasers are a widely used means for cutting steel, stainless steel, aluminum, titanium, plastic wood and other substances. They are also used for welding. In many cutting applications high pressure gas, known as assist gas, is used to aid in the cutting process. Oxygen under pressure is used to cut carbon or mild steel. Nitrogen assist gas is commonly used to cut stainless steel and argon and helium are used to cut titanium. Gases used to cut substances other than mild steel tend to be used under higher pressures than oxygen used to cut mild steel. The assist gases perform various functions depending upon the material being cut.
Oxygen reacts with the iron in mild steel and releases energy which causes more rapid melting of the steel than the laser alone would cause at the same power level. The reaction of oxygen with iron effectively doubles the energy Output of the laser. Stainless steel is cut with inert assist gas because oxygen cutting leaves an undesirable dark adherent scale on the surface and produces rough edges on thicker pieces. Due to the absence of the oxygen-iron reaction, higher power must be used to cut stainless steel.
The high viscosity of stainless steel melted by CO.sub.2 laser cutting causes dross adherence to the bottom edge of the material. To solve this problem, assist gases under very high gas pressures are blown through the kerf to drive off the dross. Assist gases may also be used to blow the "spatter" generated by the cutting process away from the laser's focusing lens and for cooling the focusing lens. Assist gas is introduced to the cutting area through the cutting head of the laser and is delivered to the cutting area by the same nozzle through which the laser beam passes. When the assist gas is introduced into the cutting head it is in the same chamber in which the focusing lens is situated and the gas therefore applies pressure to the lens.
The smoke and steel particles arising out of CO.sub.2 laser cutting and welding processes are known as "spatter". Spatter can damage a laser focusing lens or the anti-reflective coating that is deposited on the lens to enhance the transmission of energy through the lens. Spatter tends to be small particles which adhere to the focusing lens or the anti-reflective coating on the lens. Spatter can burn and pit the lens and the coating. The energy that a focusing lens transmits decreases as spatter damage and contamination build up on the lens, until the point is reached that insufficient energy is transmitted to the focal point of the laser for the efficient cutting to continue. At that point the lens must be replaced. CO.sub.2 laser focusing lenses are quite expensive. If the lens is not replaced, it begins to absorb excessive amounts of energy which causes overheating and can melt the lens. If the lens melts, the expensive internal optical components of the laser are exposed to contamination which can ruin them.
Many laser cutting systems which employ assist gases also use the assist gases to cool the focusing lens.
Recently, users of certain CO.sub.2 laser systems have employed protective windows in front of the focusing lens to protect the focusing lens from spatter. These protective windows are normally added by the user and, to date, no means of using them is provided by any CO.sub.2 laser system manufacturer for cutting applications which employ gas under high pressure. However, users with systems with sufficient room in the focusing lens mount have placed protective windows in front of the lens with limited success. Many users have laser systems in which the lens mount will not accommodate a protective window unless the focusing lens is moved. But, moving the lens alters the focal point of the laser beam. If the system does not provide the focusing flexibility to accommodate movement of the focusing lens or if there is not sufficient room in the lens mount for another optic, then the protective window cannot be inserted without some modification or addition to the lens mount that will accommodate a protective window. For this reason many users of laser welding and cutting systems have been prevented from using protective windows.
NaCl and KCl are the materials that are most practical to use as protective windows. Unfortunately, protective windows made from these materials will break under pressure. Accordingly, the use of protective windows has been largely confined to CO.sub.2 laser welding systems where assist gases are not used and pressure on the protective window is not a factor. Furthermore, when the lens within a system is intended to be cooled by assist gas the placement of a protective window in front of the lens blocks the assist gas from cooling the lens. As higher laser powers are increasingly employed in the industry, lenses are subjected to more energy which generates increased heat and the effect of heat on focusing lenses has become a matter of increasing concern.
Since most cutting lasers operate at high pressure at least part of the time, a means must be devised to deal with the pressure to make use of protective windows practical in cutting applications. Merely making room for the protective window in the lens mounting system does not deal with the pressure. Furthermore, inserting a protective window can interfere with assist gas cooling of the focusing lens.
As indicated above, the typical protective window used in a CO.sub.2 laser is a crystal material made of NaCl or KCl. These window materials are economical and they will transmit 90% or more of the available energy in a CO.sub.2 laser system without an expensive anti-reflective coating, but these materials will not withstand the high pressure generated by assist gases during cutting operations.
As noted above, there are no CO.sub.2 laser cutting systems on the market made by any major laser manufacturer that accommodate protective windows. The employment of a method or apparatus that allows a protective window to operate in a pressure neutral environment or is otherwise protected from assist gas pressure is constrained by the geometry and other design features of industrial lasers in which it is desirable to employ a protective window. Certain laser systems employ lens mounting configurations which do not leave enough room to employ means for use of protective windows in the presence of high assist gas pressures, and other laser systems employ proprietary designs incorporating features such as automatic focusing that make it impractical to alter or add to the manufacturer's lens mount design. According to a recent survey published by Industrial Laser Review, there are many thousands of industrial laser welding and cutting systems in the field and 1200 new cutting systems are being sold annually. Since none of the laser systems mentioned provide means for use of protective windows and since the cost of replacing damaged focusing lenses in such systems is material, it would be a significant advance in the art to provide a method and device for protecting focusing lenses in such laser systems from spatter through the use of protective windows which can accommodate high assist gas pressures and provide for the proper cooling of the focusing lenses.
Apparatus for holding protective windows in front of laser focusing lens is known in the art. Cosmescu (U.S. Pat. No. 5,312,397) and Diakuzno (U.S. Pat. No. 4,592,353) disclose devices intended to protect the optics of lasers employed in surgical processes from blood and body fluids that would impair their operation during procedures such as laser laparoscopy. These devices are employed in systems which are entirely different than industrial CO.sub.2 laser cutting systems. The devices disclosed by Cosmescu and Diakuzno are used in low power lasers. The device disclosed by Diakuzno relates to fiber optic systems for Nd: YAG and Argon lasers, not high power industrial lasers such as CO.sub.2 lasers.
More important, the blood and body fluids that foul the optics in a surgical procedure are considerably different from such substances as molten metals and molten plastics that attack the optics in a CO.sub.2 laser cutting system. The latter are mostly solids which do permanent damage by adhering to the lens or burning and pitting the lens, whereas the former are mostly water and can be wiped off. The economical protective windows that are practical to use for CO.sub.2 cutting lasers, such as NaCl and KCl, could not be used in surgery because they are water soluble; whereas in a surgical procedure an expensive insoluble crystal such as sapphire can be used and reused as a protective window. It would not be practical to use sapphire in a cutting laser because it is considerably more costly than NaCl and KCl. In fact, sapphire is just as costly as Zinc Selenide (ZnSe), the material from which focusing lenses are made. It makes no economic sense to sacrifice a sapphire protective window for a ZnSe lens in an industrial laser. Although a sapphire protective window could be cleaned and reused in the surgical setting, it would be quickly destroyed in an industrial cutting or welding setting. Furthermore, Sapphire is not a useful transmitter of laser at 10.6 microns, which is the wavelength at which CO.sub.2 lasers operate.
Another significant difference between high power CO.sub.2 laser cutting systems and the low power surgical laser systems in which the devices disclosed by the prior art are intended to be used, is the use of assist gas at pressures of 30 psig or higher to cut stainless steel and other substances in the CO.sub.2 laser cutting systems. High pressure assist gas is not used to cut human tissue during surgery. The high pressure assist gas used in CO.sub.2 laser cutting systems is not addressed by Cosmescu or Diakuzno. The use of inexpensive materials as protective windows in CO.sub.2 laser cutting systems requires use of a method of employing an optical mounting device or an apparatus that neutralizes assist gas pressure to provide support for the protective window such as the method and apparatus of the present invention. Furthermore, if a protective window blocks the assist gas which is intended to be a means of cooling the focusing lens, then alternative means must be adopted to allow the assist gas to cool the lens as well as the protective window.
Finally, the devices disclosed in the prior art are intended to be incorporated into systems and apparatus that are designed to accommodate protective windows whereas one aspect of the present invention concerns the retrofitting of protective windows into existing systems as well as those incorporated into new designs.